Logical positivism and logical empiricism, which together formed neopositivism, was a movement in Western philosophy whose central thesis was verificationism, a theory of knowledge which asserted that only statements verifiable through empirical observation are cognitively meaningful. The movement flourished in the 1920s and 1930s in several European centers.

Efforts to convert philosophy to this new "scientific philosophy", shared with empirical sciences' best examples, such as Einstein's general theory of relativity, sought to prevent confusion rooted in unclear language and unverifiable claims.[1]

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Logical positivists culled from Ludwig Wittgenstein's early philosophy of language the verifiability principle or criterion of meaningfulness. As in Ernst Mach's phenomenalism, whereby the mind knows only actual or potential sensory experience,[2] verificationists took all sciences' basic content to be only sensory experience. And some influence came from Percy Bridgman's musings that others proclaimed as operationalism, whereby a physical theory is understood by what laboratory procedures scientists perform to test its predictions. In verificationism, only the verifiable was scientific, and thus meaningful (or cognitively meaningful), whereas the unverifiable, being unscientific, were meaningless "pseudostatements" (just emotively meaningful). Unscientific discourse, as in ethics and metaphysics, would be unfit for discourse by philosophers, newly tasked to organize knowledge, not develop new knowledge.

Logical positivism is sometimes stereotyped as forbidding talk of unobservables, such as microscopic entities or such notions as causality and general principles,[citation needed] but that is an exaggeration. Rather, most neopositivists viewed talk of unobservables as metaphorical or elliptical: direct observations phrased abstractly or indirectly. So theoretical terms would garner meaning from observational terms via correspondence rules, and thereby theoretical laws would be reduced to empirical laws. Via Bertrand Russell's logicism, reducing mathematics to logic, physics' mathematical formulas would be converted to symbolic logic. And via Russell's logical atomism, ordinary language would break into discrete units of meaning. Rational reconstruction, then, would convert ordinary statements into standardized equivalents, all networked and united by a logical syntax. A scientific theory would be stated with its method of verification, whereby a logical calculus or empirical operation could verify its falsity or truth.

In the late 1930s, logical positivists fled Germany and Austria for Britain and the United States. By then, many had replaced Mach's phenomenalism with Otto Neurath's physicalism, whereby science's content is not actual or potential sensations, but instead is entities publicly observable. And Rudolf Carnap, who had sparked logical positivism in the Vienna Circle, had sought to replace verification with simply confirmation. With World War II's close in 1945, logical positivism became milder, logical empiricism, led largely by Carl Hempel, in America, who expounded the covering law model of scientific explanation. Logical positivism became a major underpinning of analytic philosophy,[3] and dominated philosophy in the English-speaking world, including philosophy of science, while influencing sciences, but especially social sciences, into the 1960s. Yet the movement failed to resolve its central problems,[4][5][6] and its doctrines were increasingly criticized, most trenchantly by W. V. O. Quine, Norwood Hanson, Karl Popper, Thomas Kuhn, and Carl Hempel.

In Germany, Hegelian metaphysics was a dominant movement, and Hegelian successors such as F H Bradley explained reality by postulating metaphysical entities lacking empirical basis, drawing reaction in the form of positivism.[12] Starting in the late 19th century, there was a "back to Kant" movement. Ernst Mach's positivism and phenomenalism were a major influence.

Carnap and other Vienna Circle members, including Hahn and Neurath, saw need for a weaker criterion of meaningfulness than verifiability.[13] A radical "left" wing—led by Neurath and Carnap—began the program of "liberalization of empiricism", and they also emphasized fallibilism and pragmatics, which latter Carnap even suggested as empiricism's basis.[13] A conservative "right" wing—led by Schlick and Waismann—rejected both the liberalization of empiricism and the epistemological nonfoundationalism of a move from phenomenalism to physicalism.[13] As Neurath and somewhat Carnap posed science toward social reform, the split in Vienna Circle also reflected political views.[13]

Both Moritz Schlick and Rudolf Carnap had been influenced by and sought to define logical positivism versus the neo-Kantianism of Ernst Cassirer—the then leading figure of Marburg school, so called—and against Edmund Husserl's phenomenology. Logical positivists especially opposed Martin Heidegger's obscure metaphysics, the epitome of what logical positivism rejected. In the early 1930s, Carnap debated Heidegger over "metaphysical pseudosentences".[14] Despite its revolutionary aims, logical positivism was but one view among many vying within Europe, and logical positivists initially spoke their language.[14]

In 1739, Hume cast a fork aggressively dividing "relations of ideas" from "matters of fact and real existence", such that all truths are of one type or the other.[15][16] By Hume's fork, truths by relations among ideas (abstract) all align on one side (analytic, necessary, a priori), whereas truths by states of actualities (concrete) always align on the other side (synthetic, contingent, a posteriori).[15] At any treatises containing neither, Hume orders, "Commit it then to the flames, for it can contain nothing but sophistry and illusion".[15]

Early, most logical positivists proposed that all knowledge is based on logical inference from simple "protocol sentences" grounded in observable facts. In the 1936 and 1937 papers "Testability and meaning", individual terms replace sentences as the units of meaning.[13] Further, theoretical terms no longer need to acquire meaning by explicit definition from observational terms: the connection may be indirect, through a system of implicit definitions.[13] (Carnap also provides an important, pioneering discussion of disposition predicates.)[13]

The logical positivists' initial stance was that a statement is "cognitively meaningful" only if some finite procedure conclusively determines its truth.[17] By this verifiability principle, only statements verifiable either by their analyticity or by empiricism were cognitively meaningful. Metaphysics, ontology, as well as much of ethics failed this criterion, and so were found cognitively meaningless. Moritz Schlick, however, did not view ethical or aesthetic statements as cognitively meaningless.[18]Cognitive meaningfulness was variously defined: having a truth value; corresponding to a possible state of affairs; naming a proposition; intelligible or understandable as are scientific statements.[19]

Ethics and aesthetics were subjective preferences, while theology and other metaphysics contained "pseudostatements", neither true nor false. This meaningfulness was cognitive, although other types of meaningfulness—for instance, emotive, expressive, or figurative—occurred in metaphysical discourse, dismissed from further review. Thus, logical positivism indirectly asserted Hume's law, the principle that is statements cannot justify ought statements, but are separated by an unbridgeable gap. A J Ayer's 1936 book asserted an extreme variant—the boo/hooray doctrine—whereby all evaluative judgments are but emotional reactions.

In an important pair of papers in 1936 and 1937, "Testability and meaning", Carnap replaced verification with confirmation, on the view that although universal laws cannot be verified they can be confirmed.[13] Later, Carnap employed abundant logical and mathematical methods in researching inductive logic while seeking to provide an account of probability as "degree of confirmation", but was never able to formulate a model.[20] In Carnap's inductive logic, every universal law's degree of confirmation is always zero.[20] In any event, the precise formulation of what came to be called the "criterion of cognitive significance" took three decades (Hempel 1950, Carnap 1956, Carnap 1961).[13]

The second edition of A J Ayer's book arrived in 1946, and discerned strong versus weak forms of verification. Ayer concluded, "A proposition is said to be verifiable, in the strong sense of the term, if, and only if, its truth could be conclusively established by experience", but is verifiable in the weak sense "if it is possible for experience to render it probable".[22] And yet, "no proposition, other than a tautology, can possibly be anything more than a probable hypothesis".[22] Thus, all are open to weak verification.

Upon the global defeat of Nazism, and the removal from philosophy of rivals for radical reform—Marburg neo-Kantianism, Husserlian phenomenology, Heidegger's "existential hermeneutics"—and while hosted in the climate of American pragmatism and commonsense empiricism, the neopositivists shed much of their earlier, revolutionary zeal.[1] No longer crusading to revise traditional philosophy into a new scientific philosophy, they became respectable members of a new philosophy subdiscipline, philosophy of science.[1] Receiving support from Ernest Nagel, logical empiricists were especially influential in the social sciences.[23]

In DN model, the stated phenomenon to be explained is the explanandum—which can be an event, law, or theory—whereas premises stated to explain it are the explanans.[26] Explanans must be true or highly confirmed, contain at least one law, and entail the explanandum.[26] Thus, given initial conditions C1, C2 . . . Cn plus general laws L1, L2 . . . Ln, event E is a deductive consequence and scientifically explained.[26] In DN model, a law is an unrestricted generalization by conditional proposition—If A, then B—and has empirical content testable.[27] (Differing from a merely true regularity—for instance, George always carries only $1 bills in his wallet—a law suggests what must be true,[28] and is consequent of a scientific theory's axiomatic structure.[29])

By the Humean empiricist view that humans observe sequence of events, not cause and effect—as causality and causal mechanisms are unobservable—DN model neglects causality beyond mere constant conjunction, first event A and then always event B.[24] Hempel's explication of DN model held natural laws—empirically confirmed regularities—as satisfactory and, if formulated realistically, approximating causal explanation.[26] In later articles, Hempel defended DN model and proposed a probabilistic explanation, inductive-statistical model (IS model).[26] DN model and IS model together form covering law model,[26] as named by a critic, William Dray.[30] (Derivation of statistical laws from other statistical laws goes to deductive-statistical model (DS model).)[31]Georg Henrik von Wright, another critic, named it subsumption theory,[32] fitting the ambition of theory reduction.

Logical positivists were generally committed to "Unified Science", and sought a common language or, in Neurath's phrase, a "universal slang" whereby all scientific propositions could be expressed.[33] The adequacy of proposals or fragments of proposals for such a language was often asserted on the basis of various "reductions" or "explications" of the terms of one special science to the terms of another, putatively more fundamental. Sometimes these reductions consisted of set-theoretic manipulations of a few logically primitive concepts (as in Carnap's Logical Structure of the World (1928)). Sometimes, these reductions consisted of allegedly analytic or a priori deductive relationships (as in Carnap's "Testability and meaning"). A number of publications over a period of thirty years would attempt to elucidate this concept.

After the Second World War's close in 1945, key tenets of logical positivism, including its atomistic philosophy of science, the verifiability principle, and the fact/value gap, drew escalated criticism. It was clear that empirical claims cannot be verified to be universally true.[13] Thus, as initially stated, the verifiability criterion made universal statements meaningless, and even made statements beyond empiricism for technological but not conceptual reasons meaningless, which would pose significant problems for science.[21][34][35] These problems were recognized within the movement, which hosted attempted solutions—Carnap's move to confirmation, Ayer's acceptance of weak verification—but the program drew sustained criticism from a number of directions by the 1950s. Even philosophers disagreeing among themselves on which direction general epistemology ought to take, as well as on philosophy of science, agreed that the logical empiricist program was untenable, and it became viewed as self-contradictory.[36] The verifiability criterion of meaning was itself unverified.[36] Notable critics were Nelson Goodman, Willard Van Orman Quine, Norwood Hanson, Karl Popper, Thomas Kuhn, J L Austin, Peter Strawson, Hilary Putnam, and Richard Rorty.

In 1958, Norwood Hanson's Patterns of Discovery undermined the division of observation versus theory,[38] as one can predict, collect, prioritize, and assess data only via some horizon of expectation set by a theory. Thus, any dataset—the direct observations, the scientific facts—is laden with theory.

Popper finds virtue in metaphysics, required to develop new scientific theories. And an unfalsifiable—thus unscientific, perhaps metaphysical—concept in one era can later, through evolving knowledge or technology, become falsifiable, thus scientific. Popper also found science's quest for truth to rest on values. Popper disparages the pseudoscientific, which occurs when an unscientific theory is proclaimed true and coupled with seemingly scientific method by "testing" the unfalsifiable theory—whose predictions are confirmed by necessity—or when a scientific theory's falsifiable predictions are strongly falsified but the theory is persistently protected by "immunizing stratagems", such as the appendage of ad hoc clauses saving the theory or the recourse to increasingly speculative hypotheses shielding the theory.

Popper's scientific epistemology is falsificationism, which finds that no number, degree, and variety of empirical successes can either verify or confirm scientific theory. Falsificationism finds science's aim as corroboration of scientific theory, which strives for scientific realism but accepts the maximal status of strongly corroborated verisimilitude ("truthlikeness"). Explicitly denying the positivist view that all knowledge is scientific, Popper developed the general epistemology critical rationalism, which finds human knowledge to evolve by conjectures and refutations. Popper thus acknowledged the value of the positivist movement, driving evolution of human understanding, but claimed that he had "killed positivism".

With his landmark, The Structure of Scientific Revolutions, Thomas Kuhn critically destabilized the verificationist program, which was presumed to call for foundationalism. (Actually, even in the 1930s, Otto Neurath had argued for nonfoundationalism via coherentism by likening science to a boat (Neurath's boat) that scientists must rebuild at sea.[40]) Although Kuhn's thesis itself was attacked even by opponents of neopositivism, in the 1970 postscript to Structure, Kuhn asserted, at least, that there was no algorithm to science—and, on that, even most of Kuhn's critics agreed.

Powerful and persuasive, Kuhn's book, unlike the vocabulary and symbols of logic's formal language, was written in natural language open to the layperson.[41] Ironically, Kuhn's book was first published in a volume of Encyclopedia of Unified Science—a project begun by logical positivists—and some sense unified science, indeed, but by bringing it into the realm of historical and social assessment, rather than fitting it to the model of physics.[41] Kuhn's ideas were rapidly adopted by scholars in disciplines well outside natural sciences,[41] and, as logical empiricists were extremely influential in the social sciences,[23] ushered academia into postpositivism or postempiricism.[41]

The "received view" operates on the correspondence rule that states, "The observational terms are taken as referring to specified phenomena or phenomenal properties, and the only interpretation given to the theoretical terms is their explicit definition provided by the correspondence rules".[12] According to Hilary Putnam, a former student of Reichenbach and of Carnap, the dichotomy of observational terms versus theoretical terms introduced a problem within scientific discussion that was nonexistent until this dichotomy was stated by logical positivists.[42] Putnam's four objections:

Something is referred to as "observational" if it is observable directly with our senses. Then an observation term cannot be applied to something unobservable. If this is the case, there are no observation terms.

With Carnap's classification, some unobservable terms are not even theoretical and belong to neither observation terms nor theoretical terms. Some theoretical terms refer primarily to observation terms.

Reports of observation terms frequently contain theoretical terms.

A scientific theory may not contain any theoretical terms (an example of this is Darwin's original theory of evolution).

Putnam also alleged that positivism was actually a form of metaphysical idealism by its rejecting scientific theory's ability to garner knowledge about nature's unobservable aspects. With his "no miracles" argument, posed in 1974, Putnam asserted scientific realism, the stance that science achieves true—or approximately true—knowledge of the world as it exists independently of humans' sensory experience. In this, Putnam opposed not only the positivism but other instrumentalism—whereby scientific theory is but a human tool to predict human observations—filling the void left by positivism's decline.

Logical positivism's fall reopened debate over the metaphysical merit of scientific theory, whether it can offer knowledge of the world beyond human experience (scientific realism) versus whether it is but a human tool to predict human experience (instrumentalism).[47][48] Meanwhile, it became popular among philosophers to rehash the faults and failures of logical positivism without investigation of it.[49] Thereby, logical positivism has been generally misrepresented, sometimes severely.[50] Arguing for their own views, often framed versus logical positivism, many philosophers have reduced logical positivism to simplisms and stereotypes, especially the notion of logical positivism as a type of foundationalism.[50] In any event, the movement helped anchor analytic philosophy in the English-speaking world, and returned Britain to empiricism. Without the logical positivists, who have been tremendously influential outside philosophy, especially in psychology and social sciences, intellectual life of the 20th century would be unrecognizable.[14]

^Smith, L.D. (1986). Behaviorism and Logical Positivism: A Reassessment of the Alliance. Stanford University Press. p. 314. ISBN9780804713016. LCCN85030366. The secondary and historical literature on logical positivism affords substantial grounds for concluding that logical positivism failed to solve many of the central problems it generated for itself. Prominent among the unsolved problems was the failure to find an acceptable statement of the verifiability (later confirmability) criterion of meaningfulness. Until a competing tradition emerged (about the late 1950's), the problems of logical positivism continued to be attacked from within that tradition. But as the new tradition in the philosophy of science began to demonstrate its effectiveness—by dissolving and rephrasing old problems as well as by generating new ones—philosophers began to shift allegiances to the new tradition, even though that tradition has yet to receive a canonical formulation.

^Bunge, M.A. (1996). Finding Philosophy in Social Science. Yale University Press. p. 317. ISBN9780300066067. LCCNlc96004399. To conclude, logical positivism was progressive compared with the classical positivism of Ptolemy, Hume, d'Alembert, Compte, John Stuart Mill, and Ernst Mach. It was even more so by comparison with its contemporary rivals—neo-Thomisism, neo-Kantianism, intuitionism, dialectical materialism, phenomenology, and existentialism. However, neo-positivism failed dismally to give a faithful account of science, whether natural or social. It failed because it remained anchored to sense-data and to a phenomenalist metaphysics, overrated the power of induction and underrated that of hypothesis, and denounced realism and materialism as metaphysical nonsense. Although it has never been practiced consistently in the advanced natural sciences and has been criticized by many philosophers, notably Popper (1959 [1935], 1963), logical positivism remains the tacit philosophy of many scientists. Regrettably, the anti-positivism fashionable in the metatheory of social science is often nothing but an excuse for sloppiness and wild speculation.

^"Popper, Falsifiability, and the Failure of Positivism". 7 August 2000. Archived from the original on 7 January 2014. Retrieved 30 June 2012. The upshot is that the positivists seem caught between insisting on the V.C. [Verifiability Criterion]—but for no defensible reason—or admitting that the V.C. requires a background language, etc., which opens the door to relativism, etc. In light of this dilemma, many folk—especially following Popper's "last-ditch" effort to "save" empiricism/positivism/realism with the falsifiability criterion—have agreed that positivism is a dead-end.CS1 maint: Unfit url (link)

^For example, compare "Proposition 4.024" of Tractatus, asserting that we understand a proposition when we know the outcome if it is true, with Schlick's asserting, "To state the circumstances under which a proposition is true is the same as stating its meaning".

^Examples of these different views can be found in Scheffler's Anatomy of Inquiry, Ayer's Language, Truth, and Logic, Schlick's "Positivism and realism" (reprinted in Sarkar 1996 and Ayer 1959), and Carnap's Philosophy and Logical Syntax.

^ abFetzer, James (2012). Edward N. Zalta, ed. "Carl Hempel". The Stanford Encyclopedia of Philosophy (Summer 2012 ed.). It would fall to Hempel to become perhaps the most astute critic of that movement and to contribute to its refinement as logical empiricism... Hempel himself attained a certain degree of prominence as a critic of this movement... The analytic/synthetic distinction and the observational/theoretical distinction were tied together by the verifiability criterion of meaningfulness... By this standard, sentences that are non-analytic but also non-verifiable, including various theological or metaphysical assertions concerning God or The Absolute, qualify as cognitively meaningless. This was viewed as a desirable result. But, as Hempel would demonstrate, its scope was far too sweeping, since it also rendered meaningless the distinctively scientific assertions made by laws and theories... The analytic/synthetic distinction took a decided hit when the noted logician, Willard van Orman Quine, published "Two Dogmas of Empiricism" (1953), challenging its adequacy... While the analytic/synthetic distinction appears to be justifiable in modeling important properties of languages, the observational/theoretical distinction does not fare equally well. Within logical positivism, observation language was assumed to consist of names and predicates whose applicability or not can be ascertained, under suitable conditions, by means of direct observation... Karl Popper (1965, 1968), however, would carry the argument in a different direction by looking at the ontic nature of properties... Hempel (1950, 1951), meanwhile, demonstrated that the verifiability criterion could not be sustained. Since it restricts empirical knowledge to observation sentences and their deductive consequences, scientific theories are reduced to logical constructions from observables. In a series of studies about cognitive significance and empirical testability, he demonstrated that the verifiability criterion implies that existential generalizations are meaningful, but that universal generalizations are not, even though they include general laws, the principal objects of scientific discovery. Hypotheses about relative frequencies in finite sequences are meaningful, but hypotheses concerning limits in infinite sequences are not. The verifiability criterion thus imposed a standard that was too strong to accommodate the characteristic claims of science and was not justifiable... Both theoretical and dispositional predicates, which refer to non-observables, posed serious problems for the positivist position, since the verifiability criterion implies they must be reducible to observables or are empirically meaningless... The need to dismantle the verifiability criterion of meaningfulness together with the demise of the observational/theoretical distinction meant that logical positivism no longer represented a rationally defensible position. At least two of its defining tenets had been shown to be without merit. Since most philosophers believed that Quine had shown the analytic/synthetic distinction was also untenable, moreover, many concluded that the enterprise had been a total failure. Among the important benefits of Hempel's critique, however, was the production of more general and flexible criteria of cognitive significance... Hempel suggested multiple criteria for assessing the cognitive significance of different theoretical systems, where significance is not categorical but rather a matter of degree... The elegance of Hempel's study laid to rest any lingering aspirations for simple criteria of cognitive significance and signaled the demise of logical positivism as a philosophical movement. Precisely what remained, however, was in doubt. Presumably, anyone who rejected one or more of the three principles defining positivism—the analytic/synthetic distinction, the observational/theoretical distinction, and the verifiability criterion of significance—was not a logical positivist. The precise outlines of its philosophical successor, which would be known as "logical empiricism", were not entirely evident. Perhaps this study came the closest to defining its intellectual core. Those who accepted Hempel's four criteria and viewed cognitive significance as a matter of degree were members, at least in spirit. But some new problems were beginning to surface with respect to Hempel's covering-law explication of explanation and old problems remained from his studies of induction, the most remarkable of which was known as "the paradox of confirmation".

^John Vicker (2011). Edward N Zalta, ed. "The problem of induction". The Stanford Encyclopedia of Philosophy (Fall 2011 ed.). This initial formulation of the criterion was soon seen to be too strong; it counted as meaningless not only metaphysical statements but also statements that are clearly empirically meaningful, such as that all copper conducts electricity and, indeed, any universally quantified statement of infinite scope, as well as statements that were at the time beyond the reach of experience for technical, and not conceptual, reasons, such as that there are mountains on the back side of the moon. These difficulties led to modification of the criterion: The latter to allow empirical verification if not in fact then at least in principle, the former to soften verification to empirical confirmation.

^Uebel, Thomas (2008). Edward N. Zalta, ed. "Vienna Circle". The Stanford Encyclopedia of Philosophy (Fall 2008 ed.). What Carnap later called the "liberalization of empiricism" was underway and different camps became discernible within the Circle... In the first place, this liberalization meant the accommodation of universally quantified statements and the return, as it were, to salient aspects of Carnap's 1928 conception. Everybody had noted that the Wittgensteinian verificationist criterion rendered universally quantified statements meaningless. Schlick (1931) thus followed Wittgenstein's own suggestion to treat them instead as representing rules for the formation of verifiable singular statements. (His abandonment of conclusive verifiability is indicated only in Schlick 1936a.) A second element that began to do so soon was the recognition of the problem of the irreducibility of disposition terms to observation terms... A third element was that disagreement arose as to whether the in-principle verifiability or support turned on what was merely logically possible or on what was nomologically possible, as a matter of physical law etc. A fourth element, finally, was that differences emerged as to whether the criterion of significance was to apply to all languages or whether it was to apply primarily to constructed, formal languages. Schlick retained the focus on logical possibility and natural languages throughout, but Carnap had firmly settled his focus on nomological possibility and constructed languages by the mid-thirties. Concerned with natural language, Schlick (1932, 1936a) deemed all statements meaningful for which it was logically possible to conceive of a procedure of verification; concerned with constructed languages only, Carnap (1936-37) deemed meaningful only statements for whom it was nomologically possible to conceive of a procedure of confirmation of disconfirmation.
Many of these issues were openly discussed at the Paris congress in 1935. Already in 1932 Carnap had sought to sharpen his previous criterion by stipulating that those statements were meaningful that were syntactically well-formed and whose non-logical terms were reducible to terms occurring in the basic observational evidence statements of science. While Carnap's focus on the reduction of descriptive terms allows for the conclusive verification of some statements, his criterion also allowed universally quantified statements to be meaningful, provided they were syntactically and terminologically correct (1932a, §2). It was not until one of his Paris addresses, however, that Carnap officially declared the meaning criterion to be mere confirmability. Carnap's new criterion required neither verification nor falsification but only partial testability so as now to include not only universal statements but also the disposition statements of science... Though plausible initially, the device of introducing non-observational terms in this way gave rise to a number of difficulties which impugned the supposedly clear distinctions between logical and empirical matters and analytic and synthetic statements (Hempel 1951). Independently, Carnap himself (1939) soon gave up the hope that all theoretical terms of science could be related to an observational base by such reduction chains. This admission raised a serious problem for the formulation of a meaning criterion: how was one to rule out unwanted metaphysical claims while admitting as significant highly abstract scientific claims?

^Popper then denies that science requires inductive inference or that it actually exists, although most philosophers believe it exists and that science requires it [Samir Okasha, The Philosophy of Science: A Very Short Introduction (NY: OUP, 2002), p 23],

Salmon, Wesley and Wolters, Gereon (ed.) Logic, Language, and the Structure of Scientific Theories: Proceedings of the Carnap-Reichenbach Centennial, University of Konstanz, 21–24 May 1991, Pittsburgh: University of Pittsburgh Press, 1994.

Sarkar, Sahotra (ed.) The Emergence of Logical Empiricism: From 1900 to the Vienna Circle. New York: Garland Publishing, 1996.